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Knowledge Is Key
For Intelligent Decisions
Satellite Logic is a leading,
authoritative source of information in
the Satellite Industry. Located in the
heart of the Silicon Valley, Satellite
Logic provides one of the most
valuable and comprehensive
knowledge bases on the Satellite
market! This is a primary Worldwide
information center which enables our
clients to analyze, evaluate, inquire
and select their best tailored
solutions. Our company sets the
industry standards for targeted
buying leads, reflecting a dramatic
advance over traditional marketing
solutions.
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Ever wondered how did your email landed on your inbox
through a satellite? Ever thought of the connection that
managed to download this page to your computer through a
satellite system? How does the satellite internet connection
work? Well, let's have a look at the various components.
The large central hub station
The hub station is a large satellite dish, typically 6m to 32m
diameter. The large size means high receive gain and sensitivity
and this minimises the transmit power and dish size required by
the remote customer terminals. The large size also means high
transmit gain and this reduces the transmitter power at the
hub, where a high speed outlink carrier, similar to a satellite
digital TV type carrier, needs to be transmitted. High speed
means anything from 2 to 60 Mbits/s. The transmit Hub
Common Equipment (HCE) consists of a router to interface to
the external ISP network, a DVB-IP encapsulator to embed the
IP data into an MPEG-2 format, a DVB multiplexer, a continuous
modulator, a timing clock, an up-converter and high power
amplifier.
The receive HCE consists of a low noise amplifier, down-
converter, timing and distribution MF-TDMA demodulator unit
and multiple MF-TDMA demodulator units for each inbound
carrier to be received simultaneously. Each demodulator
receives an inbound link from a large number of remote
RCSTs. The number of optional demodulators depends on the
number of inbound links.
The satellite
To work well for small dish transmit services the satellite uplink
needs to have high sensitivity. This is most readily achieved if
the uplink beam coverage area is small. Also the satellite
transponder has to have a sufficiently high gain setting. This is
not technically difficult but does need to be specified before
satellite construction starts. The gain of a satellite may be
adjusted in orbit by remote control of a gain step attenuator.
For large dish services a lower gain setting is attractive.
Frequency bands
The frequency bands can be any of C band 4/6 GHz, Ku band
10-12/14 GHz or higher (Ka band). Ku band is most popular. C
band has merit in tropical areas with heavy rainfall. The higher
Ka bands are rather unused and futuristic; there is more
bandwidth available but the technology and rain margin
required are negative factors.
Transmission technique for the outlink
A large outlink carrier from the hub is shared amongst all
the customer terminals. The bit rate is up to 60 Mbits/s rate
and is ETSI-compliant (EN-301210) for modulation and FEC. For
a particular bit rate both need the approx the same power
from the satellite but 8-PSK concentrates the power into half
the bandwidth. If you have a really powerful satellite and
perhaps slightly larger remote terminals, then 8-PSK becomes
feasible with a doubling of the satellite transponder bit rate
capacity. The carrier is formed by a series of symbols where
each symbol has 4 or 8 possible states, thus conveying 2 or 3
binary bits per symbol.
Return Channel Satellite Terminals (RCST) outdoor unit (ODU)
The outdoor ODU equipment comprises a parabolic antenna
reflector, feed, ortho-mode transducer, filters and transmit /
receive radio frequency modules. The reflector collects the
received downlink from the satellite. The larger the size the
better. Most dishes used are of offset front fed parabolic shape
with the feed at the bottom on an arm. The beam comes off
the dish at an upwards angle. Reflectors vary:
The dish size must be large enough to receive the signal,
according to site location in the satellite coverage beam.
Further, in some cases, the next larger size may be selected if a
high transmit bit rate is requested as there is a trade off
between say doubling the dish area and doubling the transmit
power amplifier size. A larger dish rather than a more powerful
transmitter is my technical preference as this gives an
improvement in the receive margin as a bonus. The reflector
surface must be accurate and not suffer from surface
irregularities or from overall bending distortion.
The feed is where the radio signal emerges from the radio
equipment and into the air. Its function is to distribute the
power across the dish area. The feed must be at the focus of
the parabolic dish shape. Behind the feed there is an ortho-
mode transducer (OMT) which separates the two polarisations
which are linear and at right angles. One polarisation is used
for transmit and the other for receive. Filters are inserted to
avoid the transmitter interfering with the receiver and finally
the receiver and transmitter are attached. The receiver module
is called a low noise block down converter (LNB), similar to that
used for digital satellite television. Two coaxial cables with F
type connectors join from the outdoor to indoor units. These
cables, one for transmit and one for receive, each carry DC
power and radio signals.
Dish pointing
Find your site latitude and longitude by looking at a map or
using a GPS receiver. Note the satellite orbit location (longitude
above the equator) that you will be using. Set the feed
polarisation angle. Set the elevation angle really accurately
using an inclinometer (allowing for the dish offset) and then,
using a compass, swing the antenna boldly around the
required azimuth direction. You should find the satellite in a
few seconds. Refine the pointing by making small offsets in all
directions, which should all cause reductions. Peak up the
polarisation with the assistance of the hub who will monitor
your transmit carrier for cross polarisation quality.
Return Channel Satellite Terminal (RCST) LinkStar indoor unit
The RCST is an integrated unit with connections for the two
coax cables to the antenna and a CAT5 10/100 Base-T
connection for an ethernet cable direct to the customers
computing equipment. There is also the mains power input.
Put the indoor box in a cool, clean, dry place. Apart from the
initial set up the box is entirely controlled from the hub.
The received MPEG-2 stream is recovered from the outbound
signal by an integrated circuit consisting of a DVB-S
demodulator and de-multiplexer. This logic demodulates the
outbound signal and the demux recovers the IP packets
intended for the specific customer terminal are then delivered
to the external network via the ethernet interface.
Latency
Satellites are a long way up and the speed of the radio waves
(same as speed of light) means that there is some delay or
latency, approx quarter of a second, associated with an end
to end satellite transmission.
Safety
The dish assembly should be mounted securely with care so
that it does not fall down on anyone. Take great care if working
on a ladder or on a roof. The view upwards from the dish to the
satellite should be clear of obstruction and the possibility of
people, vehicles, animals, trees etc getting in the way of the
beam. The radio power transmitted out of the feed is
comparable to that from several cell phones and the head,
eyes etc should not be put at the feed opening or between
the feed and the dish. The beam up to the sky, which is like a
torch beam approximately the same diameter as the dish, is
also to be avoided, although the power density here is very
much less than immediately at the feed opening.
Connecting your PC or local area network
The least complex installation will be a single PC connected
with an ethernet cable. You just need to set the IP address
and subnet mask. More commonly, a local router will be used
to enable many computers on a local area network, cafe or
wireless LAN to be connected.
Network Management
Management and control functions are performed by the NMS
(Network Management System). It is a web-based design, user-
friendly and flexible and allows Configuration, Control,
Performance, and Alarm Management, as well as Accounting,
and User Administration. It provides for the download of
software to the remote terminals.
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